I. Field of the Invention
This invention is concerned with double layer capacitors and, more particularly, with improved electrodes for the capacitor.
II. Description of the Prior Art
Double layer capacitors are disclosed in prior U.S. patents including: U.S. Pat. Nos. 3,536,963, 3,652,902, 4,327,400, 4,394,713.
In general, the above patents describe double layer capacitors which comprise a pair of polarization electrodes having a separating medium therebetween. The electrodes are composed of a solid and liquid phase and the electric double layer which characterizes these capacitors is formed at the interface between the solid and liquid (electrolyte) phases of the electrodes. The separating medium acts as an electronic insulator between the electrodes, but is sufficiently porous to permit ion migration therethrough.
A double layer capacitor is therefore made up ot two half-cells assembled on either side of a separator element. Each half-cell comprises an electrode element, a horizontally enclosing cell wall for the electrode element and a conductor element contacting one end wall of the electrode element. The cell wall and conductor element, when assembled, thus contact and confine the electrode element on the side and one end thereof.
Double layer capacitors can be made in miniature size, yet they exhibit very large capacitance when compared with conventional capacitors of similar or near similar size.
In the description herebelow, reference will be made to the "leakage current" of capacitors. "Leakage current" is defined as the internal mechanism by which the capacitor self-discharges. It is measured by determining the current required to maintain the capacitor at a given charging voltage.
Where charged capacitors are to be stored for varying periods prior to use, the leakage current becomes significant because this determines the "shelf life" of the unit. It would be especially valuable to have a capacitor which would reliably retain a useful voltage level for storage periods of several months or longer.
It has been the practice to heat treat double layer capacitors after assembly because such treatment has been found to be effective in reducing the leakage current. This heat treatment is relatively lengthy (20 hours or more) and expensive and it will be appreciated that elimination of the heat treatment, if possible, is highly desirable.